Electrostatic coating apparatus and method



Jan. 4, 1955 E. M. RANSBURG 2,698,814

ELECTROSTATIC COATING APPARATUS AND METHOD Filed May 1, 1952 INVENTOR.

EDWIN M. RANSBURG WWW? A rforneys United States Patent ELECTRQSTATICCOATING APPARATUS AND METHOD Edwin lv Ransburg, Indianapolis, Ind.,assignor to Ransburg Electra-Coating Corp., a corporation of IndianaApplication May 1, 1952, Serial No. 285,575

5 Claims. (Cl. 117-62) This invention relates to electrostatic coatingand more particularly to coating apparatus and coating methods to beused in electrostatic coating systems.

In a majority of electrostatic coating systems a field of electrostaticforce is set up which includes the article being coated and the atomizedcoating material particles in movement toward such article. Depositionof the atomized particles is primarily the result of electrostaticforces and the particles are given an electrostatic charge at the timethey leave the atomizing or spraying device or shortly thereafter. Wherethe articles are of conducting material, it has been the normalcommercial practice to mount the articles on a grounded conveyor andthus to make the articles themselves an electrode at ground potential.Another electrode at high electrical potential is provided, with thiselectrode sometimes being the spraying device and sometimes being aseparate electrode near or past which the spray particles pass in theirmovement toward the article.

When electrostatic coating methods are used to apply coating material toarticles made of non-conductive material, it is often extremelydifiicult to cause such articles to become a collecting electrode in theelectrostatic field so that the sprayed particles of coating materialare precipitated on the article through electrostatic forces. Varioustechniques, such as backing the article with a conducting member orheating or wetting the article so as to cause them to become conductors,have been tried with varying degrees of success.

The principal object of the present invention is to provide a method andapparatus whereby non-conductive articles can readily be coated in anelectrostatic coating system. To this end the invention contemplatesexposing the surface of the article to be coated to an electrodecarrying an electrostatic charge so as to provide such surface wth anelectrostatic charge differential relative to the charged particles.Where deposition of coating material particles is by electrostaticforces from an electrode at or near the atomizing means, positioning ofa non-conducting article in depositing relation to the field therebycreated causes the buildup of an electrostatic charge of the samepolarity as the electrode on the surface of the article and thus, beforethe desired amount of coating is deposited, causes the article to repelthe charged particles. Thus only a very thin unsatisfactory coatingcould be applied to non-conductive surfaces with ordinary electrostaticcoating systems. According to the present invention, however, means areprovided for removing charges so built up on the article surface so asto maintain a charge differential between the surface and the chargedparticles and thus to permit the application of a coating of desiredthickness. In the preferred form of invention illustrated andhereinafter to be described in detail, the articles are rotated andcarried along a conveyor past an ionizing electrode which is positionedadjacent the path of movement of the articles so that electrostaticcharges are deposited on the surface of the article confronting theelectrode. Positioned on the opposite side of the article is anatomizing means for atomizing fine particles of the coating materialtogether with means for creating a charge on the atomized particleshaving a polarity opposite to the polarity of the electrode. Asthearticles continue to rotate during the coating operation, that portionof the surface of the article confronted by the electrode is rotatedaway from the electrode and toward the atomizing means so as to bringthe charged portion of the surface into field creating relationship withthe charged particles and thus to cause their deposition thereon.Continued rotation of the article brings the coated portion again nearthe electrode to remove the charge deposited thereon during thedeposition of coating material and thus conditions such portion toreceive further coating as rotation of the article continues.

Other features and advantages of the invention will be readily apparentfrom the following description and drawings, in which:

Fig. l is a perspective view of an apparatus embodying the invention;and V Fig. 2 is a diagrammatic View serving to illustrate the manner inwhich the invention is utilized.

While one form of apparatus is illustrated in the accompanying drawings,and will be described hereafter as providing means for practicing thepresent invention, it will be understood that this is a representativeembodi' ment only. It will also be understood that forms of sprayingdevices other than the particular electrostatic atomizing device shownmay be utilized and that other apparatus may be employed foraccomplishing the purposes and for practicing the method of theinvention, and it is to be understood that other embodiments may beutilized without departing from the contemplated scope of the presentinvention and that no limitations arg :10 be implied from the specificdescription now provr e In the exemplary embodiment of the inventionillustrated in Fig. 1, there is shown a conveyor 10 for carrying aplurality of articles 11 along a predetermined path. An ionizingelectrode 12 is positioned on one side of the articles adjacent thepath, and an atomizing means 13 is also positioned adjacent the path buton the opposite side of the articles. The articles 11 are shown ascylindrical tubes which may be of Bakelite, cardboard, paper, or othersimilar non-conductive material. The atticles are suspended by means ofhooks 14 from a rotator 15 which is attached to a pendant 16 movablycarried on rollers (not shown) traveling inside of a hollow beam member17. The rotator 15 is provided with a cylindrical surface adapted tocontact a rotator bar 18 carried on brackets 19 by the beam 17 andserves, upon movement of the pendant 16 along the conveyor,'to rotatethe rotator and hence the article.

The electrode 12, as previously noted, is provided for the purpose ofconditioning the confronting .nonconductive surface for the reception ofcharged coating material particles. Not only may such conditioning beachieved initially through the action of the electrode in creating onthe confronting surface of the article a charge opposite to the chargeon the particles, but also the electrode serves to remove electrostaticcharges from the surface during the coating operation. For the latterpurpose the electrode may take many forms including that of the ionizingelectrode shown, or the electrode may be in the form of a charged rod orplate and if in such latter forms, should be spaced very closely to thearticle.

The ionizing electrode 12 shown comprises a plurality of dischargeelements 20 secured to a vertical upstanding support bar 21, with thedischarge elements being tapered to needle-like points at their outerends. The

ionizing electrode 12 is supported on a fixed column 22 of insulatingmaterial so that it can be maintained at a high potential with respectto ground. A. power Pack 23 has one terminal 24 connected by means ofthe conductor 25 to the ionizing electrode, with the opposite terminalof the power pack being connected to ground at 26. The conveyor itselfis grounded as indicated at 27. The discharge elements 2d of theionizing elecnode 12 are spaced from the surface of the article 11 at adistance which will allow the articles to be carried on the conveyorpast the outer pointed ends of the discharge elements without contactingsuch elements. Preferably, the elements 2% are positioned rather closeto the articlewith the space, of course, being sut'rlcient to prevent aspark from taking place between the electrode and the article or itssupport.

Located on the opposite side of the conveyor from the ionizing electrode12 is the atomizing means 13. While various types of atomizing means maybe utilized in the system herein disclosed, for the purpose of exemplaryembodiment the atomizing means is illustrated as being of the highlyefiicient rotating bell type such as more fully shown and described inthe copending application of Edwin M. Ransburg, filed February 13, 1950as Serial No. 143,994. As is more fully described in said application,the atomizing means comprises a bell-shaped head 30 supported on ahollow shaft with the shaft in turn being rotatably supported in ahousing 32 carried by an insulating column 33. A second power pack 34has its hot terminal 35 connected by means of the lead 36 to the housingof the atomizing means, with the opposite terminal of the power packbeing grounded as indicated at 37. Coating material is supplied to theinterior of the hollow shaft through a plastic or other insulating typetubing 38. The hollow shaft opens to the interior face of the bell androtation of the bell serves to spread the material in a thin film overthe interior surface of the bell for atomization from the edge thereof.

As will be noted from the drawings, the ionizing electrode 12 is chargedoppositely to the charge on the atomizing means. Thus, the ionizingelectrode may be positively charged while the atomizing means 13 isnegatively charged. It will also be noted that the atomizing means isspaced considerably farther from the article than is the ionizingelectrode. Such an arrangement permits proper dispersion of the coatingmaterial particles as they move toward the article.

Referring now to Fig. 2, the electrically charged ionizing electrode 12causes the deposition on the confronting surface of the article 11 of aplurality of charges, diagrammatically indicated at 40, which accumulatethereon to create an electrostatic charge over the surface of thearticle. The charge is of predetermined polarity, in the caseillustrated positive, and continued rotation of the article brings theportion of the surface so charged into confronting relationship with theatomizing means. The particles from the atomizing means are chargedoppositely to the charge on the surface of the article, in this casenegatively, and thus are attracted to the surface by the electrostaticforces created between them. Passage of the surface of the article infield creating relationship to the atomizing means tends to create onthe surface an electrostatic charge of the same polarity as theatomizing means and thus, in the example illustrated, tends to create onthe surface a negative charge such as illustrated at 41. A negativelycharged surface would, of course, tend to repel the negatively chargedparticles if re-exposed to a spray thereof but in the system shown thenegative charges are at least partially removed from the surface as thesurface again rotates past the electrode 12. It is not essential thatall negative charges be removed from the surface in order to effectsubsequent deposition, but only that a charge differential exist betweenthe surface and the particles sufficient to cause the deposition of theparticles thereon by the electrostatic forces. It is not essential thatelectrode 12 be maintained at an electrical potential at the oppositeside of ground from the spray particles in order that the apparatus beoperative, as the electrode 12 may be at ground potential or evenslightly charged on the same side of ground as the spray so long as itspotential relative to the surface of article 11 is such as to create onsaid surface an electrostatic charge differential relative to the sprayparticles sufiicient to effect deposition of the particles. As anotheralternative, head 13 may be maintained at ground potential in whichevent the electrode 12 should be at a high potential, either positive ornegative, in order to apply a charge to the surface of article 11 sothat an electrostatic field will be established between said surface andhead 13 which will be effective to obtain electrostatic deposition ofthe spray particles as the surface is moved into coating position.

It can be seen that the ionizing electrode may serve not only initiallyto create a charge on the surface of the article, but also, where thearticle is rotated several times during deposition, serves to removenegative charges from the surface which have accumulated thereon duringthe deposition of the charged particles, thus to leave the surface, asit is again rotated into depositing relationship, with an electrostaticcharge differential relative to the charged particles.

A single electrode and atomizing means may be sufiicient to completelyand uniformly coat the surface of an article made of non-conductivematerial. If the article, while positioned generally between theelectrode 12 and atomizing means 13, is moved slowly along the conveyoror such movement is stopped, multiple coats may be applied by rotatingthe article through multiple revolutions while in such intermediateposition. If the article is moved more rapidly along the conveyor, itmay be necessary to provide additional electrodes 12 spaced along thepath of article movement in order that the proper charge differential becreated on the surface each time the article rotates in order thatmultiple coats may be applied, and in such cases it may also be founddesirable to provide additional atomizing means along the path.

It will be clear that each time the article rotates to position aportion of its surface in confronting relationship to an electrode 12 acharge differential relative to the charged particles will be created onthe surface of the article, which subsequently serves to attract theparticles to the surface as the surface is again rotated into depositingrelationship with the charged particles. Thus multiple revolutionsproduce multiple coats. By so utilizing the apparatus, multiple thincoats may be applied to produce an ultimate coating of the desiredthickness. Such a multiple layer coating may, with some coatingmaterials, produce a better over-all coating.

While there has been shown and described an exemplary embodiment of theinvention, no unnecessary limitations of the scope of the invention areintended from such detailed description, such scope being indicated inthe following claims.

I claim:

1. The method of coating a non-conductive surface of an article in anelectrostatic coating system which comprises exposing the surface to aspray of coating material particles carrying an electrostatic charge todeposit on said surface at least a portion of said particles, moving thesurface carrying the deposited particles out of said spray and intoconfronting relationship with an ionizing discharge electrode carryingan electrostatic charge opposite to the charged particles to remove fromsaid surface at least a portion of the electrostatic charge appliedthereon during the deposition of coating material particles and tocreate on said surface an electrostatic charge differential relative tothe charge on the particles of sufiicient strength as to be capable ofelectrostatically attracting said particles to and depositing them onsaid surface, and then moving said surface away from said electrode andback into said spray of coating material particles again to depositthereon particles of the coating material.

2. The method of coating a non-conductive surface of an article in anelectrostatic coating system which comprises rotating the article to becoated, depositing on the surface of the rotating article electrostaticcharges of predetermined polarity from a stationary ionizing dischargeelectrode confronting a portion of the non-eonductive surface of thearticle whereby to accumulate on said confronting surface anelectrostatic charge, continuing to rotate the article to expose thesurface so charged to a spray of coating material particles carrying acharge relatively opposite to the charged surface, the chargedifferential between the particles and the surface being of suflicientstrength as to be capable of electrostatically attracting said particlesto and depositing them on said surface, then continuing said rotation toreturn said surface into confronting relationship with said elec trodeagain to provide on said surface an electrostatic charge differentialrelative to the particle charge, and then continuing said rotation toagain expose the charged surface to said spray of coating material.

3. The method of coating a non-conductive surface of an article in anelectrostatic coating system which comprises moving the article into aposition intermediate a stationary eletcrode maintained at high electricpotential and a spray of coating material particles carrying anelectrostatic charge relatively opposite to the electrode and rotatingthe article while in such intermediate position to expose said surfaceto said spray a plurality of times and to bring said surface intoconfronting relationship With the electrode intermediate the exposuresto remove therefrom at least a portion of the electrostatic chargedeposited thereon by the particles and to create thereon anelectrostatic charge differential relative to the charge of theparticles of sufficient strength as to be capable of electrostaticallyattracting said particles to and depositing them on said surface.

4. Apparatus for electrostatically coating a non-conductive surface ofan article comprising means for moving the article along a predeterminedpath, an ionizing discharge electrode positioned adjacent to and spacedfrom said path, means for maintaining the electrode at high electricpotential of predetermined polarity, electrostatic atomizing meansadjacent said path for atomizing coating material and including anotherelectrode from creating an electrostatic charge on the atomizedparticles of opposite polarity to the ionizing discharge electrode, andmeans for rotating the article repeatedly to bring said surfacealternately into confronting relationship with the ionizing dischargeelectrode to create between said surface and the atomized particles anelectrostatic charge differential of sufl'icient strength as to becapable of electrostatically attracting said particles to and depositingthem on said surface and then with the atomizing means.

5. Apparatus for electrostatically coating a non-conductive surface ofan article comprising a conveyor for moving the article along apredetermined path, means on the conveyor for rotating the article as itis moved along said path, an ionizing discharge electrode located infixed position closely adjacent to and on one side of said path, meansfor maintaining the electrode at high electric potential, atomizingmeans adjacent to said path and positioned oppositely to the electrode,and another electrode spaced from said path a greater distance than thefirst mentioned electrode for creating an electrostatic charge on theatomized particles of opposite polarity to the ionizing dischargeelectrode to create between the atomized particles and said surface anelectrostatic charge differential effective to deposit the particles onsaid surface, said rotating means being adapted to rotate the article aplurality of times as it passes between said electrodes repeatedly tobring said surface alternately into confronting relationship with saidionizing discharge electrode and then with the other electrode.

References Cited in the file of this patent 'UNTIED STATES PATENTS2,187,306 Formhals Jan. 16, 1940 2,221,338 Wintermute Nov. 12, 19402,287,837 Smyser June 30, 1942 2,334,648 Ransburg Nov. 16, 19432,562,358 Huebner July 31, 1951 2,604,870 Blood July 29, 1952

1. THE METHOD OF COATING A NON-CONDUCTIVE SURFACE OF AN ARTICLE IN ANELECTROSTATIC COATING SYSTEM WHICH COMPRISES EXPOSING THE SURFACE TO ASPRAY OF COATING MATERIAL PARTICLES CARRYING AN ELECTROSTATIC CHARGE TODEPOSIT ON SAID SURFACE AT LEAST A PORTION OF SAID PARTICLES, MOVING THESURFACE CARRYING THE DEPOSITED PARTICLES OUT OF SAID SPRAY AND INTOCONFRONTING RELATIONSHIP WITH AN IONIZING DISCHARGE ELECTRODE CARRYINGAN ELECTROSTATIC CHARGE OPPOSITE TO THE CHARGED PARTICLES TO REMOVE FROMSAID SURFACE AT LEAST A PORTION OF THE ELECTROSTATIC CHARGE APPLIEDTHEREON DURING THE DEPOSITION OF COATING MATERIAL PARTICLES AND TOCREATE ON SAID SURFACE AN ELECTROSTATIC CHARGE DIFFERENTIAL RELATIVE TOTHE CHARGE ON THE PARTICLES OF SUFFICIENT STRENGTH AS TO BE CAPABLE OFELECTROSTATICALLY ATTRACTING SAID PARTICLES TO AND DEPOSITING THEM ONSAID SURFACE, AND THEN MOVING SAID SURFACE AWAY FROM SAID ELECTRODE ANDBACK INTO SAID SPRAY OF COATING MATERIAL PARTICLES AGAIN TO DEPOSITTHEREON PARTICLES OF THE COATING MATERIAL.